It is said that peptic ulcer occurs when the balance between the attacking factors to gastric mucosa and defensive factors for gastric mucosa is lost. Inhibition of gastric juice secretion which is one of the attacking factors is useful for prevention and therapy of gastric ulcer. Hitherto, as drugs effective for inhibition of gastric juice secretion, anticholinergic agents and histamine H.sub.2 receptor antagonistic agents such as cimetidine etc., have been widely employed in clinic. However, when a histamine H.sub.2 receptor antagonistic agent has been used for therapy for a long period, recurrence of ulcer during interrupted administration of the drug is a serious problem. Though the recurrence of ulcers is considered to be due to decreased defensive factors at the site of gastric mucosa, its relationship with Helicobacter pylori has been recently indicated. Accordingly, an excellent anti-ulcer agent is desired, which strongly inhibits gastric juice secretion, i.e., an attacking factor, protects gastric mucosa and has an excellent antibacterial activity against Helicobacter pylori.
As pyrrolopyridazine derivatives having a gastric juice secretion inhibiting activity and gastric mucosa protective activity, a compound shown below, for example, has been known (WO 91/17164, WO 92/06979, WO 93/08190 etc.). However, its effects are not sufficient, and it has been desired to develop a compound having more potent activity. ##STR2##